Part of Bertha's seal, which project manager Chris Dixon points to during a news conference on Feb. 11, has apparently been compromised.

Elaine ThompsonAP

Contractors working for the state of Washington are planning a high-stakes operation to rescue Bertha — the world's largest tunneling machine.

Bertha is supposed to be boring a 2-mile highway tunnel under downtown Seattle, but it got stuck in December.

Bertha is on Seattle's waterfront, between South Main and South Jackson streets, about 60 feet straight down. At first, they thought the machine was being stymied by a big glacial rock. Then attention focused on the chewed-up remains of a metal pipe. But now it seems Bertha's ailment is mechanical.

"The problem with Bertha is located right behind the cutter head, which is the very front of the machine," says geotechnical engineer Joseph Wartman.

At the University of Washington, where Wartman is an associate professor, he points out the key parts of a scale model tunneling machine. The cutter head here looks somewhat like a cheese grater. On Bertha, the cutter head is five stories tall — a world record. The problem lies in the bearing that rotates that cheese grater: Apparently, some sand got in there.

"Usually there are seals placed to try to keep sand out of the bearings," Wartman says, "because of course the sand, as you might imagine, is going to wear the bearings down. But it has somehow got into the bearings, and likely through some kind of failure in the seal system itself."

That's bad. How the obstruction happened is still a matter of inquiry — and probably litigation — down the road. Some fingers are pointing at Hitachi Zosen, the Japanese company that built Bertha. It reportedly found a similar problem during testing, and the fix may not have been good enough. That company is now devising the rescue plan, which is no simple task.

"This isn't like a problem where you pull your car to the side of the road, get out, open the hood take a look at the engine," Wartman says. "The front of the tunnel-boring machine is pressed up against soil, and the tunnel-boring machine is not only underground, it's underwater."

Still, it could be worse. The good news is the breakdown happened here on the waterfront, says Chris Dixon, project manager for Seattle Tunnel Partners, the main contractor on the tunnel.

"As unfortunate as this event has been, we're very fortunate that it occurred where it did, and we're able to access it from the surface," Dixon says.

There's a vacant lot here — enough room to dig the 120-feet-deep pit needed to expose Bertha and lift out the parts that need replacing. If the machine had broken down farther along — under, say, the skyscrapers of downtown Seattle — digging down would not be an option. They wouldn't be able to back the machine up either.

One person close to the project says they probably could have still fixed it, but he says it would sort of be like a dentist doing a root canal without direct access to the patient's mouth.